The aspnet_wp or ASP.NET Machine Account is created when the
Microsoft .Net Framework 1.1 is installed onto a Windows XP computer. The user is created to run the asp.net worker process used in Microsoft's Internet Information Services, which allows
ASP.net to run on your local web server (This is pretty much its only use, it is not used to run normal .net managed executables). There is not a need to worry about this user's presence; it was not created in malicious way.
ASP.net is a programming framework built on the common language runtime (Microsoft .net framework) which can be used on a server to build powerful Web applications. For more information on ASP.net
see here